A number of consumer, commercial and military applications require improved power sources for portable electronic equipment. Traditional batteries suffer from a number of limitations. In most applications, batteries are either too heavy or do not last long enough. Recharging times for reusable batteries are very slow (typically hours), and in fact the highest energy density batteries are single-use. Most battery chemistries also use heavy metals and other toxic materials, which present safety and health risk, as well as environmental disposal complications.
Electricity needed for portable electronic devices is typically generated using batteries. Existing batteries have low energy density, and as a result, they are too heavy or do not last sufficiently long.
Power generation utilizing hydrocarbons offers a promising alternative to traditional batteries. The energy density of hydrocarbons is significantly higher than that of batteries (approximately 40 for fuels vs. 0.5 MJ/kg for lithium-ion battery). A hydrocarbon-based device with an overall efficiency of approximately 1% or greater can therefore lead to improvements over current battery technology. Furthermore, hydrocarbon-based power systems can be continuously recharged simply by physical addition of more fuel.
Proton exchange membrane (PEM)-based fuel cells convert hydrogen directly into electricity. However, systems utilizing compressed H2 produce low system-level energy density because of the high strength tanks required to store the gas at high pressures, and prevention of explosions is a major concern. Direct methanol fuel cells (DMFCs) suffer from crossover of methanol from the anode to the cathode, which depresses the cell voltage and results in fuel loss. System energy density is also reduced, since most DMFC devices require significant aqueous dilution of the methanol fuel. Solid oxide fuel cells are also being explored for the same objective but work at high temperatures creating issues with signature and have inherent safety issues due to gas phase chemistry that can lead to explosions.
Recent efforts have attempted to utilize combustion of hydrocarbons in miniature devices to directly produce heat or power. These devices typically utilize conventional homogeneous (gas-phase) combustion. A major disadvantage of homogeneous combustion is that operating temperatures are necessarily very high (>1500° C.). These high temperatures greatly limit material selection, burner life time, require extensive combustor insulation, and lead to significant NOx production and emissions. These high temperatures also present significant challenges for system compatibility with electronics, packaging, and personnel, and create thermal signatures that are undesirable for military applications. Another disadvantage of homogeneous combustion is that flames quench when confined between walls that are less than 1 mm apart, making it difficult to stabilize and maintain the reaction for long periods of time. These gap sizes can lead to combustors that are relatively bulky in size. A final disadvantage of homogeneous micro combustion devices is that many of them consist of complicated miniature parts, which are difficult and expensive to fabricate, and so far have exhibited efficiencies well below 1%.
An alternative to homogeneous microcombustion is to combust the fuel catalytically, without the production of a flame. When implemented in miniature devices, catalytic microcombustion has the potential to fully utilize the high energy densities of hydrocarbon fuels, but at much lower operating temperatures. Additionally, catalytic systems are typically easier to start, more robust to heat losses, and self-sustained at leaner fuel/air ratios. Finally, since catalytic combustion can be sustained in much smaller channels, catalytical microcombustors can potentially be designed into more compact geometries than homogeneous combustors.
Reaction rates increase with increasing catalyst surface area for a given reactor volume. Because of this, high surface areas reactors are constructed. These typically involve particulates in packed (fixed) bed and fluidized bed reactors. High surface areas provide leaner sustainable combustion, easier startup, potentially higher energy efficiency, and no emissions of unburned hydrocarbons or carbon monoxide. However, most of these particulate bed combustors require pumping the fuel-air mixture directly through the bed, which causes large pressure drops that limit overall system efficiency.
Herein we define microcombustors as devices with one or more physical dimensions being below one millimeter. We define catalytic combustion as combustion that does not involve flames. We use the terms combustor and reactor interchangeably. The equivalence ratio is defined as the fuel/air ratio normalized by the fuel/air ratio at stoichiometric conditions.
Devices have been previously described in U.S. Pat. Nos. 6,062,210, 6,497,571 B1, 6,541,676, 6,613,972, 6,710,311 B2, 6,747,178 B1, 6,770,471 B2, and 6,786,716 B1 that are microscale catalytic combustors that have a low pressure-drop, and low catalytic surface area. The catalytic surface area available is approximately that of the geometric surface area of the exposed catalyst support. The device according to the invention has a much higher specific surface area, which can lead to increased performance, especially at high flow rates.
In U.S. Pat. Nos. 6,750,596 B2 and 6,806,624 B2 devices are described that produce electrical power in microelectromechanical systems with moving parts. The device according to the invention requires no moving parts, and is therefore much simpler to fabricate and is likely to be more robust and durable.
In U.S. Pat. Nos. 4,651,019, 4,773,847, 5,599,181, 5,753,383, 5,968,456, 6,307,142 B1, 6,410,842 B1, 6,367,261 B1, 6,393,824 B1, 6,458,478 B1, 6,653,005 B1, and 6,830,596 B1 devices are described that consist of mesoscale and macroscale generation of electricity using thermoelectric devices. The thermoelectric devices utilize heat generated from combustion processes. These devices are different from those according to the invention because they are relatively large in size, and could not be easily miniaturized to a compact and portable geometry.
In U.S. Pat. Nos. 3,969,149, 5,824,947, 6,207,887, 6,560,167 B1, 6,625,990 B2, 6,670,539 B2, 6,717,043 B2, 6,787,691 B2, 6,872,879 B2 devices are described that generate electrical power utilizing microscale thermoelectric devices. However, these devices only consist of the thermoelectric, without an integrated heat source. If power is to be generated, they must be combined in some fashion with a thermal gradient generator. In contrast, our device according to the invention consists of an integral heat source and thermoelectric device.
In Schaevitz, Franz et al. (A combustion-based EMS thermoelectric power generator, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany; 2001) a microelectromechanical microcombustor-thermoelectric was fabricated. However, it had low catalyst surface area, and suffered from very low efficiencies (0.02%). In Yoshida, Kobayashi et al. (Micro-thermoelectric generator using catalytic combustor; Int. Workshop on Power MEMS, Tsukuba, Japan; 2002) a microcombustor and a thermoelectric device were fabricated and tested separately. In Yoshida, K. S. Tanaka, et al. (High-Energy Density Miniature Thermoelectric Generator Using Catalytic Combustion, J. MEMS 195-203; 2006) a microcombustor and thermoelectric device were integrated to produce electricity. However, it had low surface area and was unable to produce power from a hydrocarbon.